From 54a23e67a2f9d1225ad3f13f55a38bb145acfaba Mon Sep 17 00:00:00 2001
From: David Feuer <David.Feuer@gmail.com>
Date: Tue, 12 Jan 2021 18:41:14 -0500
Subject: [PATCH] Use eager indexing for !?

* Make `(!?)` extract a value from the vector eagerly.
* Make the range check for `(!?)` use one comparison instead of two.
---
 Data/Vector/Generic.hs | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/Data/Vector/Generic.hs b/Data/Vector/Generic.hs
index 507bf7d5..099e0534 100644
--- a/Data/Vector/Generic.hs
+++ b/Data/Vector/Generic.hs
@@ -241,8 +241,13 @@ infixl 9 !?
 -- | O(1) Safe indexing
 (!?) :: Vector v a => v a -> Int -> Maybe a
 {-# INLINE_FUSED (!?) #-}
-v !? i | i < 0 || i >= length v = Nothing
-       | otherwise              = Just $ unsafeIndex v i
+-- Lengths are never negative, so we can check  0 <= i < length v
+-- using one unsigned comparison.
+v !? i | (fromIntegral i :: Word) >= fromIntegral (length v)
+       = Nothing
+       | otherwise
+         -- Use basicUnsafeIndexM @Maybe to perform the indexing eagerly.
+       = basicUnsafeIndexM v i
 
 -- | /O(1)/ First element
 head :: Vector v a => v a -> a